Microporous polyolefin films are now in use for microfiltration films, battery separators, condenser separators and fuel cell materials, particularly for lithium-ion battery separators.
Lithium-ion batteries have been used, in recent years, for small-size electronic devices such as cellular phones and notebook personal computers, while attempts have been made to use them for electric vehicles or mini bikes applications. Lithium-ion batteries used for the latter applications are of larger size and of higher energy capacity, so much so that they require higher safety measures. Under such circumstances, a separator for lithium-ion batteries is needed which ensures the safety of the batteries even at high temperatures possibly caused by abnormal exothermic reaction in the cells, while maintaining the battery characteristics at high temperatures.
Conventionally, microporous polyethylene films have been used for separators for lithium-ion batteries. The reason for polyethylene to be used is that polyethylene not only has an excellent permeability, but is suitable for current shutdown for securing the safety of batteries. Specifically, pores in polyethylene in communication with each other are blocked up with polymer when the polymer is fused at 130° C. to 150° C., causing current shutdown, whereby the safety of batteries is secured. The term “shutdown (or shutdown temperature)” herein used means a phenomenon (or a temperature at which the phenomenon occurs) that pores of a microporous film are blocked up with fused resin, whereby the electric resistance of the film is increased, which results in the interruption of the lithium ion flow. When using a microporous film as a battery separator, the lower the shutdown temperature, the better.
Further, a microporous film must perform another function of a battery separator; in other words, it must keep its film shape, even after its pores are blocked up with polymer, so as to maintain the insulation between the cathode and anode electrodes. Thus, the higher the short-circuit temperature, the better. The term “short-circuit temperature” herein used means a temperature at which current is restored after the separator undergoes shutdown, due to the decrease in the electric resistance of the separator caused by further increase in the temperature of the separator. In conventional polyethylene separators, however, their short-circuit temperature tends to be low since their strength rapidly lowers after the crystal fusing, which can sometimes make it hard to maintain the battery safety. Thus, improvement in film strength at high temperatures is a matter of concern.
To increase the film strength at high temperatures in polyethylene battery separators, many attempts have been made in which a microporous polyethylene film and a microporous polypropylene film are stacked together.
Patent Document 1 described later discloses a stacked film that contains polyethylene and polypropylene as essential components. In the film of this patent, the shutdown temperature is obtained by: first fixing the two sides of the porous film so that the length of the film is kept constant in the oriented direction and keeping such a state for 15 minutes; and then measuring the electric resistance of the porous film at room temperature. However, in the actual batteries, temperature increase due to the exothermic reaction occurring in an abnormal situation is very rapid. Thus, when using such a film as a battery separator, the blocking of the pores is delayed since the content of polyethylene in the entire film is 2% by weight or more and 40% by weight or less, and therefore the shutdown temperature is far from sufficiently low. Further, since the microporous film is produced by stretching pore-forming process in which uniaxial stretching is performed in the low temperature region, the film is likely to crack in the direction of orientation. Particularly in large-size lithium-ion batteries, their productivity is decreased, and thus such a film is not preferable from the practical viewpoint.
Patent Document 2 described later discloses a separator produced by stacking a porous polyolefin layer having a fusing point of 158° C. or higher and a porous polyolefin layer having a fusing point of 110° C. to 150° C. The separator of this patent attains excellent heat resistance, since the polyolefin that makes up the porous polyolefin layer having a fusing point of 158° C. or higher is any one selected from the group consisting of polypropylene, poly(4-methylpentene-1), poly(3-methylbutene-1) and the copolymers or blends thereof. However, when intending to increase the permeability of the layers, the film cannot attain sufficient strength.
Patent Document 3 described later discloses a separator produced by stacking microporous films which are formed by blending polyethylene and polypropylene. In the film, as a separator, described in the document, when intending to make it thin, each of the microporous polymer films has to be made thin, since the film, as a separator, is produced by first arranging the microporous polymer films and then heating the arranged films at temperatures lower than their fusing points. This might cause film rupture or the like frequently. Further, since the microporous polymer films are formed separately, the productivity is low and the production cost is high. Still further, since the intermediate layer is also a microporous film formed by blending polyethylene and polypropylene, the permeability is hard to improve and the shutdown temperature is far from satisfactory.
As an attempt to lower the shutdown temperature, Patent Document 4 described later discloses a microporous polyolefin film produced by stacking a microporous film containing polyethylene and polypropylene, as essential components, and a microporous polyethylene film together into an integral part. The film described in the document has a low shutdown temperature and high strength at ordinary temperature at the same time, and besides, excels in permeability. However, since the content of polypropylene in the microporous film that contains polyethylene and polypropylene as essential components is low, the short-circuit temperature and film strength at high temperatures of the film are insufficient, and moreover, the high temperature storage is poor.
Patent Document 1: JP-B-3352801
Patent Document 2: JP-B-2625798
Patent Document 3: JP-A-9-259857 (U.S. Pat. No. 5,856,039)
Patent Document 4: JP-A-2002-321323